There have been several proposals to meet the ever increasing traffic demands and high quality expectations from end users for mobile broadband services. The upgrading of existing base stations to use higher data rate technologies such as High Speed Packet Access (HSPA) or Long Term Evolution (LTE), or to use other optimizations such as Multiple Input Multiple Output (MIMO), antenna tilting, etc., is one of the most widely adopted measure used to meet these demands. This may be further enhanced by a straightforward increasing of the number of base stations (e.g. E-UTRAN NodeBs or “eNBs”) in a network, which is known as “macro densification.” However, these methods of improving the data rate may provide system gains only to a certain extent, and they may end up being very expensive.
As such, the concept of heterogeneous networks, where an existing homogeneous network is overlaid with additional lower-power, low-complexity base stations, is currently being researched as a solution to mitigate the cost and/or capacity limitations of macro densification or upgrading.
The homogeneous layer of macro cells is known as a “macro” layer, as the eNBs in this layer have large coverage areas. The non-homogenous layer contains low-power nodes such as picocells and femtocells. Picocells (“picos”) include low-power eNBs and are generally suitable for indoor or outdoor usage. Femtocells (“femtos”) include home base stations (“HeNBs”) and are generally suitable for indoor home usage. A femto that is open only to few users, e.g., within a household, a shop, etc., is referred to as a Closed Subscriber Group (“CSG”) by the 3rd Generation Partnership Project (3GPP). Throughout this application, the term “small cell” is used to refer to a cell supported by a low-powered base station such as a picocell or a femtocell. Also, throughout this application any reference to an eNB is understood to refer to a macro eNB supporting a macro cell, unless the eNB is specifically referred to as a low-powered eNB supporting a small cell.
Heterogeneous networks are expected to offer a low cost alternative to macro densification and are expected to be effective, as the deployment of the low-power nodes may be made more focused towards hot spots and areas with coverage problems.
Handover is an important aspect of mobile communication systems, wherein the system tries to assure service continuity of a mobile terminal, e.g. a User Equipment or “UE”, by transferring the connection from one cell to another depending on several factors such as signal strength, load conditions, service requirements, etc. The provision of efficient/effective handovers, e.g. minimum number of unnecessary handovers, minimum number of handover failures, minimum handover delay, etc., affects not only the Quality of Service (QoS) of the end user but also the overall mobile network capacity and performance.
Currently, “s-measure”, sometimes also referred to as “s-Measure”, is used to configure when the UE starts measuring the power of neighbor cells. The s-measure is defined as a Reference Signal Received Power (RSRP) value. Once a UE's measurement of the RSRP of its serving cell drops below the s-measure threshold, the UE begins measuring the RSRP of neighboring cells, and the measured neighboring cells may ultimately be used for cell re-selection through handover. In homogenous networks, this works quite well because cells are usually deployed in such a way that they have a small shared coverage area around their cell edges. When a UE is very close to the eNB, the RSRP is typically higher than the s-measure, and when the UE moves towards the cell edge, the RSRP decreases, and at some point it falls below the s-measure and the UE starts measuring the Reference Signal (RS) from neighboring cells. Thus, the UE starts measuring RS from other cells only when it is necessary to do so.
In heterogeneous networks, on the other hand, the straightforward usage of the s-measure may cause problems, because small cells may be located close to the macro eNB (e.g. for capacity boosting in hotspots) and as such the RSRP of the serving cell may be strong enough to not fall below the s-measure threshold near the coverage area of the small cell, causing the UE not to measure the signal power of otherwise available small cells within its macro serving cell.